In building and process technology it is quite common to insulate, for instance, pipes and containers. The insulation can take place by means of e.g. pipe shells, sheets or mats. A typical insulation consists of a thermally insulating material, such as mineral wool, which on its side facing the ambient air has a protective surface layer of plastic, paper or metal.
Even if a material which is normally considered tight, such as aluminium foil, is selected for the protective surface layer and this is arranged as a continuous enclosure of the thermally insulating material, a completely tight enclosure cannot be obtained. There are always openings, such as joints between neighbouring pipe shells, longitudinal slots to allow mounting of the pipe shells on the pipes or even physical damage, through which openings water vapour can penetrate into the insulation and on to the surface that is to be insulated. Another important source of penetration of the water vapour is diffusion. Diffusion occurs also through materials that are normally considered tight, i.e. also metal or plastic foils, and thus cannot be fully prevented.
When the temperature at the insulated surface is below the dew point of the ambient air, the water vapour is condensed. The problem is that the condensate cannot evaporate, which in the long run causes damage not only to the insulated surface in the form of e.g. corrosion, but also to the actual insulation.
There are established solutions on the market, which function well and provide for removal of the condensate.
A first example is given by WO 91/18237. This solution uses layers of a hygroscopic material on both sides of a thermally insulating material which is adapted to be arranged round, for instance, a pipe. The two layers communicate with each other through an opening in the thermally insulating material, whereby condensate by capillary action can be transported from the inner layer to the outer layer. In the case involving a pipe shell, the inner layer is arranged to surround the pipe and, with its respective free ends, protrude through the slot of the pipe shell to such an extent that these ends can be arranged against the outside of the pipe shell where they are exposed to the ambient air and form an evaporation surface. The hygroscopic material can be resembled to a wick which lets the surface exposed to condensate communicate with the ambient air where the condensate can evaporate freely.
Variants of the same approach are defined in WO 95/19523 where the extent of the hygroscopic material has been reduced. Instead of covering the whole length of the pipe, the hygroscopic material is arranged, for instance, in the form of strips which are equidistantly spaced from each other along the length of the pipe. Like in WO 91/18237, the hygroscopic material extends from a direct contact with the surface in which condensation occurs, i.e. the insulated surface, to the outside of the insulation where it is exposed to the ambient air and forms an evaporation surface.
Another variant is defined in WO 94/05947, which states, inter alia, that the hygroscopic material can be a hygroscopic paint which is applied to the pipe and to selected parts of the pipe shell in such a manner that a continuous paint surface forms, extending from the surface of the pipe to the outside of the pipe shell in order to form an evaporation surface. Thus, the paint has the same function as the hygroscopic material in the above-mentioned WO 91/18237.
WO 97/16676 discloses a solution in which a first gap is formed between the pipe and the surrounding thermally insulating material. The latter is provided with a number of capillary active openings connecting its inside with its outside so that condensate by capillary action can be conducted from the first gap and the surface of the pipe to the outside of the thermally insulating material where the condensate can evaporate to the ambient air. The outside of the thermally insulating material is enclosed by a water-repellent membrane. The membrane is arranged so that a second gap is formed between the thermally insulating material and the membrane. A layer of water-absorbing material is arranged in each gap and it is preferred for the two layers to be connected with each other by, for instance, a slot in the thermally insulating material. Examples of suitable materials of the water-repellent membrane are waterproof and diffusion open textile materials.
A common feature of all prior-art solutions to the problem with removing of condensate thus is that a hygroscopic material is arranged on the surface on which condensate is formed and that this hygroscopic material is brought into direct contact with the ambient air, which takes place by the hygroscopic material forming an evaporation surface which directly or indirectly is exposed to the ambient air. This technique is very well established. However, it is associated with a certain overdimensioning as regards the amount of hygroscopic material. This overdimensioning affects the cost of the product. Moreover, a protective surface is frequently used in the form of, for instance, an adhesive tape which is arranged to partially cover that part of the hygroscopic material which is exposed on the outside of the insulation, whereby this part of the material is prevented from absorbing moisture from the ambient air in an uncontrolled manner.